The present invention relates to a method for sensing and a magnetic sensor for moving objects. More particularly, the present invention relates to a sensor for determining speed and direction of moving or rotating targets, such as, for example, gears, shafts, joints, wheels, fans, turbines, tires, conveyors or like moveable objects.
The demand for higher performance vehicles and for lightweight devices providing accurate speed and direction of rotation measurements for shafts, wheels, and gears continues to increase rapidly. As a result, improvements in electronics-based products for use in sensing applications are needed. For example, in the automotive industry, it is desirable to use sensors to accurately measure the speed and direction of rotation of wheels, transmissions, shafts, gears and other rotatable objects. This measurement information can be processed by on board computers in communication with the sensors to improve the fuel efficiency and power in automatic transmissions, to monitor the performance of the transmission or to control the automobile""s braking systems (e.g. an Anti-lock Braking System). For example, in an automobile equipped with anti-lock brakes, a computer in communication with the automobile braking sensors may receive information relating to the tire""s rotational velocity and rotational direction, process such information and thereafter control the automobile""s brakes to lessen the severity of any perceived skid or braking malfunction.
A large number of speed and direction sensing devices exist. Such devices are discussed, for example, in U.S. Pat. Nos. 5,880,585, 5,523,679, 5,264,789 and 4,789,826. Furthermore, ring magnets or linear magnets are well known in the sensing arts. A conventional ring magnet 110 is illustrated in FIGS. 1A and 1B, while a conventional linear magnet 109 is illustrated in FIGS. 1C and 1D. Ring magnet 110 is typically formed in a circular pattern, having a plurality of alternating north and south pole permanent magnet segments. This xe2x80x98chainxe2x80x99 of magnets forms a circle or ring of magnets which, for example, can be placed or disposed circumferentially around an object. Similarly, linear magnet 109 is typically formed in an approximately linear pattern, having a plurality of alternating north and south pole permanent magnet segments. For purposes of this discussion, the north polarity of each magnet is referred to in the drawings as item 113 while the south polarity is referred to in the drawings as item 112.
As seen in FIGS. 1A through 1D, a plurality of flux lines 102 are illustrated which represent an exemplary magnetic field generated by each magnet. As is known in the art, a magnetic field (represented by magnetic flux lines) exists such that the magnetic field flows from the north polarity to the south polarity regardless of the coordinate dimension used. For example, the flux lines illustrated in FIGS. 1A and 1B may depict a magnetic field which is external to the ring magnet 110, internal to the ring magnet 110, or may depict a magnetic field in any orientation (e.g., three dimensional) flowing from the magnet""s north polarity to the magnet""s south polarity. Similarly, the flux lines exemplified in FIGS. 1C and 1D may depict a magnetic field adjacent to the linear magnet 110 or may depict a magnetic field in any orientation (e.g., three dimensional) flowing from the magnet""s north polarity to the magnet""s south polarity. In each case, the flux lines flow away perpendicular (e.g., 103) from the magnet""s north polarity (e.g., 113) and magnetically curve or bend towards the magnet""s south polarity (e.g., 112) where they flow back perpendicular (e.g., 105) to the magnet""s south polarity. The area where the flux lines magnetically curve or bend can be represented by a portion of the flux line (e.g., 104) which is approximately horizontal or parallel to the magnet""s surface in any given coordinate location. Thus, for example, as seen in FIG. 1B, flux line 106 may represent a portion of a magnetic field which can be generated between any north polarity of a magnet and a south polarity of a magnet in any coordinate axis.
A Wheatstone bridge, such as the representative circuit illustrated in FIG. 2, may be helpful in determining resistance of a variable resistor, and thus, it may be useful as a sensing means in some applications. A typical Wheatstone bridge includes resistive elements RA (231), RB (232), RC (233) and RD (234), all in electrical communication with voltage source 210 and ground 240. A differential voltage VB may be measured to obtain a voltage signal that changes with changes in the resistance of each of the four resistive elements RA (231), RB (232), RC (233) and RD (234). A Wheatstone bridge such as the one illustrated in FIG. 2 has the advantage of being able to self-compensate for temperature variations in the range of, for example, xe2x88x9240xc2x0 to 200xc2x0 Centigrade. Thus, for example, a Wheatstone bridge could be used in thermocouple applications.
When a sensor is placed within a magnetic field, the sensor""s resistors may be influenced by the magnetic field. As illustrated in FIG. 3A, a magnetic field (e.g., flux lines 102) which runs substantially parallel to a resistive device 301 tends to have little or no electrical effect on the resistive device. In contrast, as illustrated in FIG. 3B, a magnetic field 102 which runs perpendicular to the resistive device 301 appears to electrically influence the resistive device and appears to change the resistivity of the device.
Prior art magnetic sensing devices suffer from many disadvantages. For example, prior art magnetic sensing devices (such as silicon Hall integrated sensors) have a limitation of around 20 gauss minimum signal due to inherent stress-induced offsets. This reduces the range of available signal which in turn reduces the available mechanical tolerance. Using two independent sensors would produce placement errors which may be very difficult to tolerate in a manufacturing environment.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can only be gained by taking the entire specification, claims, drawings, and abstract as a whole.
The method and device according to the present invention addresses many of the shortcomings of the prior art and further provides the advantage of, among other items, generating the rotational velocity and rotational direction of a rotating object. In accordance with one exemplary embodiment of the present invention, at least two bridges are provided, each having resistors. The resistors are configured to be electrically influenced by a magnetic field from an adjacent array of magnetic elements. In another embodiment, the bridges are in electrical communication with a computing means, such as a microprocessor or a microcontroller. Thus, for example, each bridge may comprise a Wheatstone bridge in communication with a microprocessor. In another embodiment, at least two bridges in communication with a computing means may be provided adjacent to a magnetic array, each bridge having resistors which are electrically influenced by a magnetic field. In another embodiment, the present invention includes at least two bridges with each bridge having a first set of resistors and a second set of resistors configured in a bridge, the first set of resistors being oriented approximately perpendicular to the second set of resistors. In another embodiment, the present invention includes at least two bridges fabricated as an integrated circuit, the bridges each comprising resistors which are electrically influenced by a magnetic field. In another embodiment, the present invention comprises at least two bridges, fabricated as an integrated circuit, with each bridge configured to be in communication with a computing means and each bridge having resistors which are electrically influenced by a magnetic field.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.